In a conventional magnetic data storage system it is known to store digital data from a host device, eg a computer. Digital data may be stored on a magnetic medium by switching the polarity of current through a magnetic write head which is in close proximity to the magnetic media. Conventionally, the magnetic media may comprise a flexible elongate tape which is coated with a magnetic material and which is wound between two reels past a magnetic write head. Alternatively, the magnetic media may also comprise a rigid disk which is coated with a magnetic medium. Data is recorded to the disk by moving a recording head in a radial direction across the disk while the disk is rotated about its centre.
In tape-based magnetic data recording systems, data may be recorded using a plurality of write heads and read with a plurality of read heads. Conventionally, these write and read heads may be either substantially stationary with respect to the rest of the device in which case data are stored in a plurality of tracks parallel to the elongate direction of the tape or the write and read heads may be mounted on a drum which is rotated about an axis at an angle to the elongate direction of the tape, in which case data is stored in a series of tracks diagonally across the magnetic tape.
Conventionally, recording heads are fabricated from ferrite which comprises a sintered combination of a ferro-magnetic material and a ceramic combined to yield a material with the high magnetic permeability of the former and the high electrical resistance of the latter. However, writing data to magnetic media using ferrite heads becomes more inefficient at high data bit-rates. At high frequencies the losses due to irreversible heating of the write head results in a roll-off of the magnetic field of the write head for a given input current. Referring to FIG. 1 herein, there is illustrated by a solid line 10 a plot indicating how the write head frequency response decreases as the frequency of the writing current increases when using a ferrite write head. The output for a given current has dropped significantly at the "roll off" frequency 120 which is, typically of the order 30-40 MHz and which limits a maximum write rate of the head to the order 80 MBits/s. When writing data to a magnetic storage medium the data pulses as written to the magnetic storage medium are convolved with the frequency response of the write head. This process of convolution has the effect of "smearing out" individual pulses on the storage medium. The smeared out pulses on the storage medium may overlap with the adjacent smeared out pulses as the pulse separation decreases. Eventually, the degree of overlap between adjacent pulses can become such that pulses cannot be distinguished during a read operation. Hence, the frequency response of the write head in a magnetic data storage device can act as a constraint on the highest frequencies and hence the highest data bit rates recorded to the magnetic data storage medium.
It is known to attempt to correct for this roll off in the frequency response of the write head by preferentially boosting high frequencies in the write current according to a response curve such as illustrated by dashed line 130 in FIG. 1. Preferentially boosting high frequencies prior to input to a write head in the write current to compensate for the decrease in efficiency of the write head should yield an approximately flat frequency response as illustrated by the dot dashed line 140 in FIG. 1. This technique of boosting the high frequencies is conventionally known as "Write Pre-Equalization" (WPE). However, write drivers in digital magnetic data storage systems are highly non-linear devices. Conventional write drivers comprise switches which send two polarities of current to the write heads in order to record two distinct magnetization states on the magnetic media. Hence, any prior art attempts to boost high frequencies in such devices have been complex. In particular, Ampex produced a write pre-equalization scheme which comprised a current driver based on a linear amplifier, i.e. the output of the write driver was proportional to the input to the write driver. The Ampex scheme applied a boost to the high frequency response of the write driver to compensate for the roll-off in the frequency response of the write head. However, the Ampex implementation of write pre-equalization required substantial power, typically of the order 15 W, and could only be produced using discrete components and hence it was not possible to implement this as a single application specific integrated circuit (ASIC). In addition, the Ampex scheme was also difficult to set up.
In addition the roll-off in the frequency response of a recording head at high frequencies as described hereinbefore there is another, more significant, effect resulting from a finite rise time of a magnetic field generated by a record head in response to a substantially step-like change in a recording current driving said record head. In response to a, for example, positive going head of a driver current the resulting magnetic field starts to increase. However, in the event of a negative going edge in the recording head driver current happening before the magnetic field in a recording head has reached a maximum steady state value then the magnetic field starts to decease and at a certain physical location on a magnetic data storage medium the magnetization of the medium changes direction. However, in the case where increase in magnetic field, in response to a positive going edge of a driver current has sufficient time to rise to a level closer to the final steady state value then the time required for the magnetic field to fall to zero in response to a negative going edge of a head driver current is longer than in the previous case described hereinabove. Hence, positions of magnetic field transitions on a magnetic data storage medium may be laterally displaced with respect to one another dependent upon a time duration between a last positive going edge of the head driver current and a last going edge of a edge driver current. This lateral displacement of magnetization direction on the magnetic recording medium is also known herein as "bit shift", "peak shift" and "transition shift" is non-linear and is affected by the duration of at least the last driver current pulse.
The effect of these non-linear lateral displacements of regions of magnetization on magnetic recording media can result in timing errors during a subsequent read operation of data stored on the storage medium resulting in an increase in the number of errors occurring during the read operation.
The ongoing pressure in the development of new magnetic data storage systems is to increase the data storage capacity of any magnetic data storage media. By increasing the effective bandwidth of the write head in a magnetic data storage system it is possible to increase the bit rate at which data are written to, for example, magnetic tapes and hence increasing the storage capacity of the tape. There is a need for a means to increase the effective bandwidth of ferrite recording heads in a way which can be implemented as an ASIC and which is straightforward to both calibrate and use.